1. Field of the Invention
This invention pertains to hydraulically powered yarder systems and particularly to hydraulically powered interlock systems where the pumps and motors operate in open loop circuits.
2. Description of the Prior Art
In conventional hydraulic open loop circuitry for logging yarders, the interlock motor on the haulback drum planetary gear set frequently becomes driven by the drum via cable load on the drum, causing the motor to act like a pump and create a fluid hydraulic backpressure which conventionally is passed through a relief valve back to the hydraulic reservoir. In closed loop hydraulic circuits, this cable load can act as a regenerative power source; but in open loop circuits, the hydraulic motor effectively serves as a braking device for maintaining tension on the cable between the haulback drum and the main drum with the energy supplied from the drum by being rotated by the cable load essentially being wasted or dissipated as heat in the hydraulic fluid.